The invention relates to a high tension vacuum switch comprising two relatively axially movable switching contacts, associated with each one of which is an annular shielding electrode which is electrically coupled with its respective switching contact and protrudes, in the opened condition of the switch and in the direction of the closing movement, slightly from the contact surface of the switching contact.
In a like vacuum switch, e.g. the switch disclosed in the U.S. Pat. No. 3,914,568, the said shielding electrode serves the purpose to create, in the opened condition of the switch in front of the contact surface of the respective switching contact, an electric field which decreases the risk of an arc discharge between the contact surfaces of both switching contacts preceding the closing of the switch. Originally these contact surfaces are smooth and they have a high voltage strength, but during operation they will become rougher owing to the arc discharge which directly precedes the closing of the switch, whereby the voltage strength decreases. The increasing roughness of the contact surfaces is not only the result of burning by the arc discharge but also of the fact that during the closing of the switch the contacts are welded one to the other and during the opening of the switch are again pulled loose from one another. By using the shielding electrode it is achieved that, after starting the closing operation of the switch, the respective switching contacts still remain some time outside the electric field coming from the other switching contact and being sufficiently strong to introduce an arc discharge, so that the switch keeps a sufficiently high voltage strength irrespective of the fact that the contact surfaces become rougher.
In order to prevent the occurrence of an arc discharge between the shielding electrodes themselves or to avoid that, during an arc discharge from a switching contact, the respective shielding electrode is hit by metal particles torn from said contact, whereby the smoothness of its surface may be affected and the risk of an arc discharge between the two shielding electrodes themselves increases it is necessary in the known vacuum switch to mount the shielding electrodes relatively far from the contact surfaces of the switching contacts.
This has the disadvantageous result that the outer dimensions of the vacuum switch become larger, the switching stroke to be effected by the driving mechanism of the switch becomes longer and the masses to be moved become greater.